A riflescope is provided with a detachable digital ocular lens group that enables vision enhancement functions, e.g. night vision, zoom, etc while being replaceable with a regular ocular group in the event of power failure or faulty components. To ensure accuracy of the digital ocular lens group, a non-visible marker, e.g. IR marker, may be placed on a reticle that can be detected by a photo-sensor of the digital ocular. A processor can use the detected position of the marker to calibrate the digital display of the digital ocular.